-Tetrahydropterin forms in the basic structure of series of analogues which serve effectively as cofactors for hydroxylase enzymes. Most analogues are formed by placement of various side chains on the 6-carbon. We recently acquired a compound which has a phenyl group attached at this 6-position. The present investigation compared this compound with tetrahydrobioptin for effective hydroxylation fo phenylamine, tyrosine and tryptophan. The enzymes were phenylalanine hydroxylase purified from rat liver and tyrosine and tryptophan hydroxylase from rat brain. In addition we examined the ability of the reductase system to continually regenerate reduced 6-methylpterin. Our kinetic analysis showed that 6-methylpterin serves as effective as the natural cofactor (tetrahydrobiopterin) for the various hydroxylases. Also, 6-methylpterin is incorporated effectively into the endogenous reductase system.